Rollup merge of #66226 - lzutao:asm-usage-link, r=Centril
Rollup merge of #66226 - lzutao:asm-usage-link, r=Centril

add link to unstable book for asm! macro

r? @Centril

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust